The present invention relates to ultrasonic transducers and, more particularly, to such transducers employing piezoelectric elements. A major objective of the present invention is to provide a small transducer providing for high resolution and relatively artifact-free ultrasound images.
To combat heart disease, a leading cause of death and disability in many countries, physicians require detailed data on the vasculature of the heart. In vivo, intravascular ultrasonic imaging offers a relatively benign method of obtaining such information. Ultrasonic imaging involves transmitting an ultrasonic acoustic wavefront pulse into a body and detecting the reflection of that pulse. Reflections occur at boundaries where acoustic impedance changes. The times at which reflections are received correspond to the depths of these impedance boundaries. By stepping a transducer through a selected angle, one can obtain a two-dimensional (angle and depth) ultrasound image that is essentially a map of impedance boundaries. The intensity and position of these impedance boundaries can then be interpreted to characterize the condition of a vessel and its immediate environment.
The quality of the image is strongly affected by its resolution, which is in turn determined by the ultrasound wavelengths used to examine a body. Shorter wavelengths, which correspond to higher frequencies, provide higher resolution images. Higher frequencies attenuate more rapidly, limiting their use for depth examinations. Accordingly, high frequency transducers are most appropriate for high-resolution near-field imaging. For example, whereas 5-20 MHz ultrasound frequencies are useful for prenatal and peripheral vessel examinations, 40 MHz and higher are desired for intravascular examinations of vessel walls.
Most intravascular and intracavity ultrasound transducers are conventional "thickness-mode" types with electrodes disposed on longitudinally (axially) opposing faces of a piezoelectric substrate. Prior to use, these transducers are "poled" by applying a strong electric potential between the electrodes, polarizing the piezoelectric substrate. In operation, a periodically varying potential difference is applied between the electrodes, causing the piezoelectric substrate to vibrate longitudinally at the alternation frequency. Typically, a back transducer face interfaces with air or a low acoustic impedance absorber-backing; a front transducer face interfaces with an impedance matching material. This configuration causes an ultrasound wavefront to be propagated longitudinally through the front face. The transducer faces can be flat; alternatively, a preset geometric focus can be implemented by using spherical front and/or rear faces.
The frequency of the ultrasound wavefront is determined by the frequency of the electrical pulses used to excite the piezoelectric material. In a conventional thickness-mode transducer, the efficiency of the transfer of the electrical energy at a given center frequency f.sub.0 into the longitudinal ultrasound wavefront is related to the thickness of the piezoelectric substrate. The relationship between resonant frequency f.sub.0 and substrate thickness t is given by f.sub.0 =V.sub.L /2t, where V.sub.L is the longitudinal velocity of an acoustic wavefront in the piezoelectric material. This resonance relationship dictates that thinner piezoelectric substrates are required for higher frequencies. For example, a 20 megaHertz (MHz) piezoelectric substrate is about 0.1 millimeters (mm) thick, while a 40 MHz is about 0.05 mm thick. The thin piezoelectric substrates required for higher frequencies are difficult to fabricate using conventional lapping and polishing techniques. Yields can be low and, therefore, costs are high.
In addition, at such small thicknesses, piezoelectric crystal grain size limits efficiency; this reduced efficiency lowers overall image intensity and therefore increases the relative noise level in the image. Bulk piezoelectric materials are commonly formed by sintering fine ferroelectric particles together. The bulk ferroelectric material is lapped and polished to define a substrate. Electrodes are disposed on opposite faces of the substrate. A voltage differential is imposed between the electrodes to pole the ferroelectric substrate so that it becomes piezoelectric. The ferroelectric particles at the substrate surfaces can be sufficiently disrupted by the lapping and polishing that they impair the piezoelectric quality of the substrate. The percentage of surface particles to all particles in a substrate increases with decreasing substrate thickness. For very thin substrates, the percentage of disrupted surface particles can significantly degrade image quality. This degradation can be reduced by using finer-grain ferroelectric particles, but remains a constraint on transducer design frequencies.
Interdigitated transducers offer a high-frequency alternative to conventional thickness-mode transducers. Interdigitated transducers have two electrodes on one face of a piezoelectric substrate. Each electrode has a number of straight-line segments that are suggestive of fingers or "digits". The straight-line segments of each electrode are interleaved with those of the other electrode; in other words, the fingers of the two electrodes are "interdigitated".
Generally, interdigitated transducers are poled by applying a strong constant potential across opposing faces, as is done with conventional thickness-mode transducers. This thickness-dimension poling results in a piezoelectric character optimized for thickness mode operation. In operation, an alternating potential is applied between the interdigitated electrodes formed on the "back" face of the transducer. This causes surface acoustic waves to be propagated. In addition, some oblique longitudinal waves are propagated.
By poling an interdigitated transducer using the interdigitated electrodes, the piezoelectric substrate can be made to alternate poling orientations on the same pitch as the interdigitated transducers. When the interdigitated transducers are excited, a nonoblique longitudinal wavefront is generated that propagates through the piezoelectric substrate and out the front face, thus emulating a conventional thickness-mode transducer. In this case, efficient energy coupling is dependent on the relation of the excitation frequency and the pitch of the interdigitated electrodes, and not on the substrate thickness. Hence, high frequency thickness-mode transducers have been provided that are not subject to the fragility and granular effects afficting conventional thickness-mode transducers. Longitudinal interdigitated electrodes are discussed by L. J. van der Pauw, "The planar transducer--a new type of transducer for exciting longitudinal acoustic waves", Applied Physics Letters, Vol. 9, No. 3, Aug. 1, 1966, pp. 129-131, and Kiyoshi Nakamura, Hirsohi Shimizu and Nobuaki Sato, "Planar transducers using PbTiO.sub.3 ceramics for short pulse ultrasound generation", 1982 Ultrasonics Symposium (A), pp. 484-497.
The higher frequencies afforded by interdigitated transducers promise higher resolution images. However, the attainment of higher resolution images increases the importance of certain image artifacts than could be overlooked at lower resolutions. One of these artifacts is related to imperfect focus. The problem with focus is aggravated with interdigitated transducers. Whereas, conventional thickness-mode transducers can be fabricated on substrates with spherical surfaces to provide a well-defined focus, it is difficult to achieve comparable focal definition using thickness-mode interdigitated transducers.
A second artifact of concern relates to distortion of the ultrasound wavefront. Typically, a circular wavefront is transmitted through a cylindrical window. A cylindrical window distorts the wavefront differently in the "steering" plane orthogonal to the windows axis than in a "azimuthal" plane orthogonal to the steering plane. This distortion renders an ultrasound image harder to interpret. Postprocessing can correct some of the distortion, but can delay image rendering. The delay can interfere with the realtime availability of the image, diminishing its value to the physician.
A third artifact is speckle, the presence of twinkling elements in an image. Twinkling image objects can be difficult to interpret. Stop action photos can catch a twinkling image object at either a bright or a dim portion of its cycle. In either case, the stop action representation is deceptive. Speckle is the result of the beating of different reflections that have undergone slightly different phase shifts upon reflection.
While focusing artifacts, distortion, and speckle plague all ultrasound images, they tend to be more salient in high resolution images. To the extent that high resolution imaging is attainable, artifact reduction becomes increasingly important. Accordingly, what is needed is a robust high-frequency ultrasound transducer suitable for intravascular and other medical applications that provides for artifact reduction.